Present control units for occupant protection systems, which, for example, include an airbag system, a seatbelt tightener, a roll bar, etc., generally have the ability to switch the occupant protection system to a so-called “sleep mode” despite the continuously applied battery voltage. Almost no functionality is required in sleep mode and power consumption may be reduced to less than a predefined value of 100 μA, for example. The decision as to whether the system should be active or “asleep” may be made by a voltage monitor, for example, on a separate input terminal pin of an airbag system ASIC (application-specific integrated circuit) in the case of an occupant protection system configured as an airbag system. The airbag system is activated as soon as an applied voltage is detected at this input terminal pin, which is higher than a predefined threshold value of 3V, for example. The airbag system remains deactivated below the predefined threshold value. This voltage threshold for the sleep detection is the default setting and cannot be selected or adjusted for a specific project. In addition to this sleep threshold, which defines the logic decision about an ON state and/or an OFF state of the airbag system, there is also a voltage threshold, which is a function of the specific embodiment for each type of airbag system, above which the full functionality of the airbag system must be ensured. A distinction is made in principle between “6 V devices” and “>6 V devices.” The “6 V devices” are characterized in that they are fully functional above a supply voltage of approximately 6 V due to optimized and more cost-intensive input filters on the power supply. The “>6 V devices” do not meet this requirement until the voltage supply is approximately 7 V.
If the supply voltage is below 6 V or 7 V, the airbag system is already partially activated by exceeding the sleep threshold of approximately 3 V, but it is impossible to ensure that the airbag system will reach a state of full readiness. Thus, a great deal of heat may develop or radiation of EMC in the airbag system may be increased. There is only the option of activating or stopping functions of the airbag system which are controlled by a software program running in the control unit. However, this does not apply to the circuit parts, which are independent of the software programs, such as the central voltage supply of the occupant protection system.
German Patent Application No. DE 102 55 433 A1 describes a method for operating a voltage supply circuit in a power saving mode, in which the power consumption is reduced in comparison with regular operation. The voltage supply circuit includes a voltage transformer having a voltage transformer switch, which is controllable for the voltage transformer operation, and a voltage transformer capacitor and an in-phase regulator downstream from the voltage transformer for supplying voltage to a consumer, in particular a microprocessor. The voltage supply circuit may be used for safety devices, for example, airbags, seatbelt tighteners or roll bars. To reduce the power consumption, instead of the regular voltage transformer operation, its voltage transformer switch is operated together with the in-phase regulator as a two-point regulator with regard to the output voltage generated at the output of the voltage transformer. The conventional two-point regulation is preferably implemented in such a way that detection means for detecting the voltage generated at the output of the first voltage transformer and control means and detection means are provided, which activate the voltage transformer switch when the detection means indicate a voltage value below the first threshold value and deactivate it when the detection means indicate a voltage value above a second voltage value. The voltage transformer switch may be triggered via a comparator, for example.